1. Field of the Invention
The present invention relates to inkjet printing methods for producing images durable for outdoor usage.
2. Description of the Related Art
The production of outdoor signage, advertising and promotional displays is shifting from conventional printing techniques, such as offset printing, to inkjet printing due to its capability of short run printing variable data in a economically viable way.
However, it has been observed that UV inkjet printed images over time suffer from weathering conditions, such as flaking off and fading of the image. This is induced by UV radiation from the sun causing chemical degradation; by changes in temperature and humidity causing tensile stress/fatigue stress; by airborne pollutants such as ozone, NOx and SOx; by sand abrasion winds; by salt water and acid rain; and by biological factors such as mildew, algae and bird's droppings. Flaking off means that printed image comes off a substrate in flakes or thin small pieces.
US 2003021961 A (3M) disclose imaged articles that are durable for outdoor usage by ink jet printing radiation curable inks to primed substrates. The primers are coated on polymeric sheets and then inkjet printed with radiation curable inkjet inks. One example using a radiation curable primer is disclosed in [0197]-[0198]. The radiation curable primer is coated on a substrate, then radiation curable inkjet ink is jetted on the uncured primer and then both the primer and the inkjet ink are cured together.
EP 2053100 A (AGFA) discloses an inkjet printing method comprising the steps of: a) providing a first radiation curable composition curable by free radical polymerization or cationic polymerization; b) applying a layer of the first radiation curable composition on a substrate; c) curing the layer; and d) jetting on the cured layer a second composition curable by a different polymerization than the first composition but selected from the group consisting of free radical polymerization and cationic polymerization characterized in that the first composition comprises a cationically polymerizable compound having at least one (meth)acrylate group in an amount of at least 25 wt % based upon the total weight of the first curable composition.
US 2006275590 (LORENZ ET AL) discloses a method of printing a durable image on a substrate comprising the steps of: coating the treated surface with an acrylic polyurethane mixture; allowing the acrylic polyurethane coated substrate to set; and then printing a UV-cured ink design on the substrate surface.
EP 1737892 A (ASHLAND) discloses the application of radiation curable overprint varnishes for printed substrates based on multifunctional, uncrosslinked, liquid Michael addition resins to improve weather resistance.
US 2002086914 (3M) discloses radiation curable ink jet inks that are printed on a variety of porous and nonporous substrates which after curing form durable, weatherable, abrasion resistant, printed images. No information is given on light fading.
There exists still a need to provide inkjet printed images having improved weather resistance especially exhibiting improvement in flaking off, light fading, flexibility and hardness.